Antioxidant Potential of Aqueous Dispersions of Fullerenes C60, C70, and Gd@C82

The antioxidant potential (capacity and activity) of aqueous fullerene dispersions (AFD) of non-functionalized C60, C70, and Gd@C82 endofullerene (in micromolar concentration range) was estimated based on chemiluminescence measurements of the model of luminol and generation of organic radicals by 2,2'-azobis(2-amidinopropane) dihydrochloride (ABAP). The antioxidant capacity was estimated by the TRAP method, from the concentration of half-suppression, and from the suppression area in the initial period. All three approaches agree and show that the antioxidant capacity of AFDs increased in the order Gd@C82 < C70 < C60. Mathematical modeling of the long-term kinetics data was used for antioxidant activity estimation. The effect of C60 and C70 is found to be quenching of the excited product of luminol with ABAP-generated radical and not an actual antioxidant effect; quenching constants differ insignificantly. Apart from quenching with a similar constant, the AFD of Gd@C82 exhibits actual antioxidant action. The antioxidant activity in Gd@C82 is 300-fold higher than quenching constants.

Green and rapid preparation of long-term stable aqueous dispersions of fullerenes and endohedral fullerenes: The pros and cons of an ultrasonic probe

A green, scalable, and sustainable approach to prepare aqueous fullerene dispersions (AFD) C60, C70, endohedral metallofullerene Gd@C82, and their derivatives C60Cl6, C70Cl10, and supramolecular and ester-like derivatives, 10 fullerene species total, is proposed. For the first time, an immersed ultrasonic probe was used to preparing dispersions for pristine fullerenes without addends. Both ultrasound-assisted solvent-exchange and direct sonication techniques for AFD preparation using an immersed probe were tested. The average time for AFD preparation decreases 10-15 times compared to an ultrasound-bath-assisted technique, while final fullerene concentrations in AFDs remained at tens of ppm (up to 80 ppm). The aqueous dispersions showed long-term stability, a negatively charged surface with a zeta potential up to -32 mV with an average nanocluster diameter of no more than 180 nm. The total anionic and cationic compositions of samples were found by inductively coupled plasma atomic emission spectroscopy and chromatographic techniques. The highlights and challenges of using an ultrasound probe for AFD production are discussed.

Non-Functionalized Fullerenes and Endofullerenes in Aqueous Dispersions as Superoxide Scavengers

Endohedral metal fullerene are potential nanopharmaceuticals for MRI; thus, it is important to study their effect on reactive oxygen species (ROS) homeostasis. Superoxide anion radical is one of the key ROS. The reactivity of aqueous dispersions of pristine (non-functionalized) fullerenes and Gd@C₈₂ endofullerene have been studied with respect to superoxide in the xanthine/xanthine oxidase chemiluminescence system. It was found that C₆₀ and C₇₀ in aqueous dispersions react with superoxide as scavengers by a similar mechanism; differences in activity are determined by cluster parameters, primarily the concentration of available, acting molecules at the surface. Gd endofullerene is characterized by a significantly (one-and-a-half to two orders of magnitude) higher reactivity with respect to C₆₀ and C₇₀ and is likely to exhibit nanozyme (SOD-mimic) properties, which can be accounted for by the nonuniform distribution of electron density of the fullerene cage due to the presence of the endohedral atom; however, in the cell model, Gd@C₈₂ showed the lowest activity compared to C₆₀ and C₇₀, which can be accounted for by its higher affinity for the lipid phase.

Efficient regioselective [4+2] cycloaddition of o-quinodimethane to C(70)(CF(3))(10)

The Diels-Alder reaction of C(1)-C(70)(CF(3))(10) and 3,6-dimethoxy-1,2-quinodimethane leads regioselectively to the formation of a new cycloadduct that has been fully characterized by spectroscopic and electrochemical methods as well as by X-ray diffraction.

Synthesis and X-ray or NMR/DFT structure elucidation of twenty-one new trifluoromethyl derivatives of soluble cage isomers of C76, C78, C84, and C90

Adding 1% of the metallic elements cerium, lanthanum, and yttrium to graphite rod electrodes resulted in different amounts of the hollow higher fullerenes (HHFs) C76-D2(1), C78-C2v(2), and C78-C2v(3) in carbon-arc fullerene-containing soots. The reaction of trifluoroiodomethane with these and other soluble HHFs at 520-550 degrees C produced 21 new C76,78,84,90(CF3)n derivatives (n = 6, 8, 10, 12, 14). The reaction with C76-D2(1) produced an abundant isomer of C2-(C76-D2(1))(CF3)10 plus smaller amounts of an isomer of C1-(C76-D2(1))(CF3)6, two isomers of C1-(C76-D2(1))(CF3)8, four isomers of C1-(C76-D2(1))(CF3)10, and one isomer of C2-(C76-D2(1))(CF3)12. The reaction with a mixture of C78-D3(1), C78-C2v(2), and C78-C2v(3) produced the previously reported isomer C1-(C78-C2v(3))(CF3)12 (characterized by X-ray crystallography in this work) and the following new compounds: C2-(C78-C2v(3))(CF3)8; C2-(C78-D3(1))(CF3)10 and C(s)-(C78-C2v(2))(CF3)10 (both characterized by X-ray crystallography in this work); C2-(C78-C2v(2))(CF3)10; and C1-C78(CF3)14 (cage isomer unknown). The reaction of a mixture of soluble higher fullerenes including C84 and C90 produced the new compounds C1-C84(CF3)10 (cage isomer unknown), C1-(C84-C2(11))(CF3)12 (X-ray structure reported recently), D2-(C84-D2(22))(CF3)12, C2-(C84-D2(22))(CF3)12, C1-C84(CF3)14 (cage isomer unknown), C1-(C90-C1(32))(CF3)12, and another isomer of C1-C90(CF3)12 (cage isomer unknown). All compounds were studied by mass spectrometry, (19)F NMR spectroscopy, and DFT calculations. An analysis of the addition patterns of these compounds and three other HHF(X) n compounds with bulky X groups has led to the discovery of the following addition-pattern principle for HHFs: In general, the most pyramidal cage C(sp(2)) atoms in the parent HHF, which form the most electron-rich and therefore the most reactive cage C-C bonds as far as 1,2-additions are concerned, are not the cage C atoms to which bulky substituents are added. Instead, ribbons of edge-sharing p-C6(X)2 hexagons, with X groups on less pyramidal cage C atoms, are formed, and the otherwise "most reactive" fullerene double bonds remain intact.

1,3,7,10,14,17,21,28,31,42,52,55-Dodeca-kis(trifluoro-meth-yl)- 1,3,7,10,14,17,21,28,31,42,52,55-dodeca-hydro-(C(60)-I)[5,6]fullerene

The title compound, C(72)F(36), is one of four isomers of C(60)(CF(3))(12) for which crystal structures have been obtained. The fullerene mol-ecule has an idealized I(h) C(60) core with the 12 CF(3) groups arranged in an asymmetric fashion on two ribbons of edge-sharing C(6)(CF(3))(2) hexa-gons, a para-meta-para-para-para-meta-para ribbon and a para-meta-para ribbon, giving an overall pmp(3)mp,pmp structure. There are no cage Csp(3)-Csp(3) bonds. The F atoms of two CF(3) groups are disordered over two positions; the site occupancy factors are 0.85/0.15 and 0.73/0.27. There are intra-molecular F⋯F contacts between pairs of CF(3) groups on the same hexa-gon that range from 2.521 (3) to 2.738 (4) Å.

Electrochemical, Spectroscopic, and DFT Study of C60(CF3)n Frontier Orbitals (n = 2−18): The Link between Double Bonds in Pentagons and Reduction Potentials

The frontier orbitals of 22 isolated and characterized C(60)(CF(3))(n) derivatives, including seven reported here for the first time, have been investigated by electronic spectroscopy (n = 2 [1], 4 [1], 6 [2], 8 [5], 10 [6], 12 [3]; the number of isomers for each composition is shown in square brackets) fluorescence spectroscopy (n = 10 [4]), cyclic voltammetry under air-free conditions (all compounds with n <or= 12), ESR spectroscopy of C(60)(CF(3))(n)- radical anions at 25 degrees C (n = 4 [1] and 10 [1]), and quantum chemical calculations at the DFT level of theory (all compounds including n = 16 [3] and 18 [2]). DFT calculations are also reported for several hypothetical C(60)(CF(3))(n) derivatives. The X-ray structure of one of the compounds, 1,6,11,16,18,26,36,41,44,57-C(60)(CF(3))(10), is reported here for the first time. Most of the compounds with n <or= 12 exhibit two or three quasi-reversible reductions at scan rates from 20 mV s(-1) up to 5.0 V s(-1), respectively. The 18 experimental 0/- E(1/2) values (vs C(60)(0/-)) are a linear function of the DFT-predicted LUMO energies (average E1/2 deviation from the least-squares line is 0.02 V). This linear relationship was used to predict the 0/- E(1/2) values for the n = 16 and 18 derivatives, and none of the predicted values is more positive than the 0/- E(1/2) value for one of the isomers of C(60)(CF(3))(10). In general, reduction potentials for the 0/- couple are shifted anodically relative to the C(60)(0/-) couple. However, the 0/- E(1/2) values for a given composition are strongly dependent on the addition pattern of the CF3 groups. In addition, LUMO energies for isomers of C(60)(X)(n) (n = 2, 4, 6, 8, 10, and 12) that are structurally related to many of the CF(3) derivatives were calculated and compared for X = CH(3), H, Ph, NH(2), CH(2)F, CHF(2), F, NO(2), and CN. The experimental and computational results for the C(60)(CF(3))(n) compounds and the computational results for more than 50 additional C(60)(X)(n) compounds provide new insights about the frontier orbitals of C(60)(X)(n) derivatives. For a given substituent, X, the addition pattern is as important, if not more important in many cases, than the number of substituents, n, in determining E(1/2) values. Those addition patterns with double bonds in pentagons having two C(sp(2)) nearest neighbors result in the strongest electron acceptors.

X-ray structure and DFT study of C1-C60(CF3)12. A high-energy, kinetically-stable isomer prepared at 500 °C

The title compound, prepared from C(60) and CF(3)I at 500 degrees C, exhibits an unusual fullerene(X)12 addition pattern that is 40 kJ mol(-1) less stable than the previously reported C(60)(CF(3))12 isomer.

Thermally Stable Perfluoroalkylfullerenes with the Skew-Pentagonal-Pyramid Pattern: C60(C2F5)4O, C60(CF3)4O, and C60(CF3)6

Reaction of C(60) with CF(3)I at 550 degrees C, which is known to produce a single isomer of C(60)(CF(3))(2,4,6) and multiple isomers of C(60)(CF(3))(8,10), has now been found to produce an isomer of C(60)(CF(3))(6) with the C(s)-C(60)X(6) skew-pentagonal-pyramid (SPP) addition pattern and an epoxide with the C(s)-C(60)X(4)O variation of the SPP addition pattern, C(s)-C(60)(CF(3))(4)O. The structurally similar epoxide C(s)-C(60)(C(2)F(5))(4)O is one of the products of the reaction of C(60) with C(2)F(5)I at 430 degrees C. The three compounds have been characterized by mass spectrometry, DFT quantum chemical calculations, Raman, visible, and (19)F NMR spectroscopy, and, in the case of the two epoxides, single-crystal X-ray diffraction. The compound C(s)-C(60)(CF(3))(6) is the first [60]fullerene derivative with adjacent R(f) groups that are sufficiently sterically hindered to cause the (DFT-predicted) lengthening of the cage (CF(3))C-C(CF(3)) bond to 1.60 A as well as to give rise to a rare, non-fast-exchange-limit (19)F NMR spectrum at 20 degrees C. The compounds C(s)-C(60)(CF(3))(4)O and C(s)-C(60)(C(2)F(5))(4)O are the first poly(perfluoroalkyl)fullerene derivatives with a non-fluorine-containing exohedral substituent and the first fullerene epoxides known to be stable at elevated temperatures. All three compounds demonstrate that the SPP addition pattern is at least kinetically stable, if not thermodynamically stable, at temperatures exceeding 400 degrees C. The high-temperature synthesis of the two epoxides also indicates that perfluoroalkyl substituents can enhance the thermal stability of fullerene derivatives with other substituents.

Synthesis and structures of poly(perfluoroethyl)[60]fullerenes: 1,7,16,36,46,49-C60(C2F5)6and 1,6,11,18,24,27,32,35-C60(C2F5)8

The high-temperature reaction of C60 and C2F5I produced poly(perfluoroethyl)fullerenes with unprecedented addition patterns.

Poly(trifluoromethyl)fullerene Radical Anions. An ESR/Vis−NIR Spectroelectrochemical Study of C60F2,4 and C60(CF3)2,10

Cyclic voltammograms are reported for C(60)(CF(3))(n) derivatives for the first time. The compounds studied were 1,9-C(60)(CF(3))(2) and 3 isomers of C(60)(CF(3))(10), including the structurally characterized derivative 1,3,7,10,14,17,23,28,31,40-C(60)(CF(3))(10) (C(60)(CF(3))(10)-3). The compound 1,9-C(60)(CF(3))(2) exhibited 3 reversible reductions; C(60)(CF(3))(10)-3 exhibited 2 reversible reductions; the other 2 isomers of C(60)(CF(3))(10) each exhibited 1 reversible reduction. ESR and near-IR spectroelectrochemical experiments were performed to characterize some of the C(60)(CF(3))(n)(-) and C(60)(CF(3))(n)(2-) species generated by cyclic voltammetry. The ESR spectrum of the C(60)(CF(3))(10)-3(-) radical anion consisted of an envelope of 25 lines centered at g = 2.0032 (the apparent a value is ca. 0.5 G), evidence of coupling between the unpaired electron and a significant number of the CF(3) fluorine atoms. The most significant finding is that this radical anion has a half-life in solution at 25 degrees C of about 7 min.

Variable-Temperature 19F NMR and Theoretical Study of 1,9- and 1,7-C60F(CF3) and Cs- and C1-C60F17(CF3): Hindered CF3 Rotation and Through-Space JFF Coupling

Milligram amounts of the new compounds 1,9- and 1,7-C60F(CF3) (ca. 85:15 mixture of isomers) and C60F3(CF3) were isolated from a high-temperature C60/K2PtF6 reaction mixture and purified to 98 mol % compositional purity by two-dimensional high-performance liquid chromatography using Buckyprep and Buckyclutcher columns. The previously observed compounds C60F5(CF3) and C60F7(CF3) were also purified to 90+ mol % for the first time. Variable-temperature 19F NMR spectra of the mixture of C60F(CF3) isomers and the previously reported mixture of C(s)- and C1-C60F17(CF3) isomers demonstrate for the first time that fullerene(F)n(CF3)m derivatives with adjacent F and CF3 substituents exhibit slow-exchange limit hindered CF3 rotation spectra at -40 +/- 10 degrees C. The experimental and density functional theory (DFT) predicted deltaH++ values for CF3 rotation in 1,9-C60F(CF3) are 46.8(7) and 46 kJ mol(-1), respectively. The DFT-predicted deltaH++ values for 1,7-C60F(CF3), C(s)-C60F17(CF3), and C1-C60F17(CF3) are 20, 44, and 54 kJ mol(-1), respectively. The (> or = 4)J(FF) values from the slow-exchange-limit 19F spectra, which vary from ca. 0 to 48(1) Hz, show that the dominant nuclear spin-spin coupling mechanism is through-space coupling (i.e., direct overlap of fluorine atom lone-pair orbitals) rather than coupling through the sigma-bond framework. The 2J(FF) values within the CF3 groups vary from 107(1) to 126(1) Hz. Collectively, the NMR data provide an unambiguous set of (> or = 4)J(FF) values for three different compounds that can be correlated with DFT-predicted or X-ray diffraction derived distances and angles and an unambiguous set of 2J(FF) values that can serve as an internal standard for all future J(FF) calculations.

Synthesis, Structure, and 19F NMR Spectra of 1,3,7,10,14,17,23,28,31,40-C60(CF3)10

A significant improvement in the selectivity of fullerene trifluoromethylation reactions was achieved. Reaction of trifluoroiodomethane with [60]fullerene at 460 degrees C and [70]fullerene at 470 degrees C in a flow reactor led to isolation of cold-zone-condensed mixtures of C60(CF3)n and C70(CF3)n compounds with narrow composition ranges: 6 < or = n < or = 12 for C(60)(CF3)n and 8 < or = n < or = 14 for C70(CF3)n. The predominant products in the C(60) reaction, an estimated 40+ mol % of the cold-zone condensate, were three isomers of C60(CF3)10. Two of these were purified by two-stage HPLC to 80+% isomeric purity. The third isomer was purified by three-stage HPLC to 95% isomeric purity. Thirteen milligrams of this orange-brown compound was isolated (5% overall yield based on C60, and its C1-symmetric structure was determined to be 1,3,7,10,14,17,23,28,31,40-C60(CF3)10 by X-ray crystallography. The CF3 groups are either meta or para to one another on a p-m-p-p-p-m-p-m-p ribbon of edge-sharing C6(CF3)2 hexagons (each pair of adjacent hexagons shares a common CF3 group). The selectivity of the C70 reaction was even higher. The predominant product was a single C70(CF3)10 isomer representing >40 mol % of the cold-zone condensate. Single-stage HPLC led to the isolation of 12 mg of this brown compound in 95% isomeric purity (27% overall yield based on converted C70. The new compounds were characterized by EI or S(8)-MALDI mass spectrometry and 2D-COSY 19F NMR spectroscopy. The NMR data demonstrate that through-space coupling via direct overlap of fluorine orbitals is the predominant contribution to J(FF) values in these and most other fullerene(CF3)n compounds.